He did by the end, but as
@Lancet Fluke said, Pellegrini basically ruined him, making him cross to Aguero standing in the box surrounded by 8 defenders for 3 years. If you watch his first season, or saw him at Sevilla he was a genuinely good crosser of the ball.
The other thing is that Pellegrini was terrible at creating space for wingers. Pep spends a lot of time isolating our wingers so they have the time and space to pick a cross, then the ball goes out to them early and they attack a box with multiple runners and only a few defenders running backwards.
Navas would often get the ball shuffled out to him when the defence was settled and the creative players had passed it around and found no gaps, have 5-10m space to try and beat a defender and get a cross in before the bye line, and it's almost impossible - we've seen similar from Mendy, Cancelo and even KDB this season when Pep's tactics have broken down.
Anway like I said not the thread for this debate, but I wish Mahrez had his work ethic!